This invention pertains to a process for continuously aging and filtering xanthated alkali cellulose (XAC) to form a spinnable viscose solution. The continuous aging and filtration process herein disclosed may be incorporated in conventional batch viscose rayon systems, or may be used in conjunction with the continuous xanthation apparatus and process disclosed in U.S. Pat. No. 4,037,039 (of common inventorship and assignment herewith).
Use of the present invention in conjunction with the continuous process disclosed in the aforementioned patent results in a completely continuous process from the start of xanthation through to spinning. The present invention can be used with current standard batch xanthation and mixing processes, by combining the batches and treating the mixture continuously thereafter.
The economic attractiveness of the process and the quality of the product are greatly enhanced by the use of a continuous process throughout.
Regardless of whether the xanthation process itself is a batch or continuous process, the resulting cellulose xanthate must be filtered to remove insoluble fiber or foreign matter. Prior to spinning, the solution must be deaerated (degassed) and aged (ripened) under controlled temperature conditions, resulting in a partial decomposition and redistribution of the xanthate groups on the cellulose molecules. These chemical reactions cause changes in the state of solution of the cellulose and are an important determinant in the characteristics of the final fiber as well as the efficiency of the spinning process.
Rate of aging is a function of temperature and the chemical composition of the solution. Since fiber properties and spinnability are both affected by the degree of aging, uniformity of aging is of paramount importance in the production of uniform fiber.
Conventional batch aging processes comprise passing viscose through a plurality of tanks maintained at about 18.degree. C. The viscose is filtered from two to four times through this passage which takes approximately 25-100 hours. For each batch movement, pumps have to be actuated in conjunction with the opening and closing of the necessary valves. On emptying each tank, an appreciable volume of viscose is left sticking to the tank because of the highly sticky, viscous nature of viscose. Furthermore, in spinning tanks it is necessary to leave a heel, because if the whole volume is pumped out, air will be reintroduced into the deaerated viscose. All this leads to non-uniformity of treatment among respective batches and possible re-formation of gels after filtration.
Further, in conventional filter systems, the life of the first filter in the series (the most upstream filter as the solution passes in the downstream direction) is usually much shorter by a factor of twenty or more than the other filters in the series due to the high initial gel content of the solution passing therethrough. The other filters become plugged or inoperable at extended cycle times, with the most downstream filter typically lasting the longest, even though the filter medium is much tighter than in earlier filters. Accordingly, the overall process is subject to frequent interruption as the various filter elements of the respective filters have to be cleaned or changed, with each changing or cleaning resulting in many economic and process disadvantages. The most serious problems caused by filter changing are (a) dislodgement into the viscose stream of gels or particles that inevitably stick to the filter plates and frames, valves, pipes, etc., (b) the economic loss of the very appreciable amount of viscose that adheres to filters and filter media, (c) the nuisance and cost of changing and cleaning filters and media, and (d) the stream pollution effect of the viscose washed out of filters and media, or what is even more expensive the neutralization and decomposition of such viscose so that it is no longer harmful to streams.
Having in mind the above-noted disadvantages of batch aging processes heretofore known, it is an object of the present invention to provide a process wherein the viscose solution is uniformly aged by continuous processing means.
With respect to the frequent process interruption caused by frequent filter element change and/or cleaning, it is also an object of the present invention to provide a filtration process wherein the filter life, averaged over all filters, is substantially extended, resulting in fewer total changes for a plant, or section of a plant.